1. Field of the Invention
The present invention relates to RF power amplifiers and mounting systems for such amplifiers. More particularly, the present invention relates to RF power amplifiers mounted on pallet structures.
2. Description of Prior Art and Related Information
RF power amplifiers require careful design of all RF signal lines to avoid RF energy loss. In particular, RF signal lines require a consistent connection to ground in order to properly propagate RF signals along the signal line. If the signal line is not properly grounded along the entire signal path, RF signals may reflect, radiate or generate heat. In addition, the distance of the path to ground is important. A longer path to ground distance increases the probability of an RF signal radiating or reflecting as opposed to a shorter path. This consideration becomes more significant at both higher power and frequencies, and in particular the frequency ranges common in modern cellular base station applications.
Conventionally, RF gain blocks are structurally arranged on a pallet which acts as a substrate and heat sink for the associated RF circuitry. These pallets are mounted in a housing base and electrical connections between the RF and DC lines on the pallet and associated traces on the base must be made. The RF connection point is most critical and is typically referred to as the RF launch. The pallet is designed to keep acceptable tolerances of the over hang of the RF gain blocks and the base to provide a consistent ground across the pallet/base connection. A screw at each RF launch is typically employed to mount the pallet to the base and provide a positive ground connection. Despite these efforts to control the pallet/base connection, existing pallet structures, while providing an RF ground, do not provide a fixed and positive RF ground, which is consistent from amplifier to amplifier and over time and temperatures.
One cause of this problem with existing RF pallet structures is that the RF ground is not consistent when moved from a test fixture to a final product. When moving the pallet to a production amplifier assembly, the RF ground changes in distance and location relative to the RF traces and RF launch for various reasons. For example, when the thinned pallet landing is torqued by the machine driving of the screw, the landing may warp. This will change the RF ground location. Also, machine tolerances may be varied in production environments.
Another cause of variations in RF ground is due to changes introduced by temperature variations. Such temperature changes will cause relative movement of the pallet and base since the pallet has the heat generating power transistors mounted thereon. Also, the pallet and base may have different thermal coefficients of expansion. Furthermore, over time thermal cycling may cause the screw at the RF launch to loosen. All these factors cause the specific RF ground location relative to the RF traces at the pallet/base connection to change in an unpredictable way.
Another important consideration in RF power amplifiers is maintaining correct phase relations between RF signal paths. This requires each path to have a consistent known reactance. A problem with existing RF pallet structures is the inherent existence of unpredictable parasitic reactances. The reactances inherent to the conventional structures are introduced because of variables such as junctions of ground contact locations and changes over temperature. The junctions typically are located in phase dependent (high RF power) locations and are susceptible to phase and amplitude errors, return loss changes and possible coupling issues. As a result, immediate negative effects to tuning are created.
Furthermore, some existing RF pallet structures require manual labor in installing pallet to base coupling mechanisms. This leads to increased cost of manufacturing.
Thus, the need exists for an RF amplifier assembly having a reliable RF pallet ground which reduces RF signal losses. A need further exists for an RF pallet design where parasitic reactances are substantially eliminated and the immediate negative effects to tuning can be minimized and compensated over temperature while a need further exists for an RF pallet design compatible with low manufacturing costs.